Impact of Mesophyll Diffusion on Estimated Global Land CO2 Fertilization

Abstract:

In C₃ plants, CO₂ concentrations drop considerably along mesophyll diffusion pathways from substomatal cavities to chloroplasts where CO₂ assimilation occurs. Global carbon cycle models have not explicitly represented this internal drawdown and so overestimate CO₂ available for carboxylation and underestimate photosynthetic responsiveness to atmospheric CO₂. An explicit consideration of mesophyll diffusion increases the modeled cumulative CO₂ fertilization effect (CFE) for global gross primary production (GPP) from 915 PgC to 1057 PgC for the period of 1901 to 2010. This increase represents a 16% correction large enough to explain the persistent overestimation of growth rates of historical atmospheric CO₂ by Earth System Models. Without this correction, the CFE for global GPP is underestimated by 0.05 PgC yr^-1ppm^-1. This finding implies that the contemporary terrestrial biosphere is more CO₂-limited than previously thought.